This invention relates in general to the manufacture of cement clinker, particularly in rotary kilns. More specifically, the invention relates to the introduction of slag into the kiln to enhance the clinker production process.
The details of a typical cement pyroprocessing operation are well known. One typical process utilizes a rotary kiln into which cement raw materials, such as limestone, clay and sand, are introduced. The kiln is inclined so that the raw materials progress along the length of the kiln from input to output end. The inclined kiln is heated at or near the output end to create a number of operating zones within the kiln or kiln system. For instance, at the input end, a pre-calcining zone can be provided that has a gas temperature of about 1000xc2x0 F. (538xc2x0 C.). The kiln gas temperature can be increased to about 1600xc2x0 F. (871xc2x0 C.) in a calcining zone where the CaCO3 in the raw materials is decomposed.
The calcined material then passes to a clinkering zone where it faces the burning zone temperature inside the kiln, approximately 2732xc2x0 F. (1500xc2x0 C.). It is in this zone that the feedstock is converted into the typical cement compounds, such as tricalcium silicate, dicalcium silicate, tricalcium aluminate, etc. A cooling zone follows at the output of the kiln.
A variety of materials have been added to the cementious compositions in the production of cement clinker. The use of slag in the production of cement dates back to 1774 when a mortar was made with ground blast-furnace slag and slaked lime. The first commercial use of blended slag cements arose in Europe in the mid-1800""s. A current example of a blended cement using a blast-furnace slag is found in U.S. Pat. No. 5,976,243. Many slags are well suited for use in clinker production because the slags can include many chemical constituents common to the cement chemical compounds. In addition, many slags can be added to the cement clinker without any deleterious effects to the cement kiln or to the clinker product. Thus, the slag, which is essentially a waste material from metal production, takes on value and can reduce the quantity of more expensive feedstock or virgin feedstock required in the clinker production.
The use of slag as a feedstock in the pyroprocessing of cement, including Portland cement, is also known. For example, U.S. Pat. No. 2,600,515, issued in 1952, discloses introducing blast-furnace slag directly into the flame in order to avoid problems associated with the fusibility of slag. The patent of Young, U.S. Pat. No. 5,421,880, describes introducing a steel slag at the feed end of the kiln to combine with the feedstock material, and then heating the mixture to form cement clinkers. This patent, and a subsequent U.S. Pat. No. 5,494,515 to the same inventor, contemplate adding a crushed and screened air-cooled slag to the lime-containing feedstock material.
As might be appreciated from the materials being processed and the extreme temperatures being utilized, cement clinker product has significant energy requirements. The manufacture of cement is second only to power generation in fossil fuel consumption for heating the kilns. Moreover, the pyroprocessing yields significant emissions, with CO2 emissions being the most prominent.
There remains a consistent need to improve the efficiency and yield of the cement clinker production process. It is desirable to optimize fuel usage and maximize clinker production.
In order to address these needs, the present invention contemplates a process in which vitrified slag material is used as feedstock at the feed end of a cement pyroprocessing operation. The vitrified slag material is combined with the lime-containing material, preferably at the feed end of the kiln, but at least prior to the clinkering zone of the kiln. The lime-containing materials can be selected to yield any type of cement, such as Portland and other hydraulic cements.
In one embodiment, the slag material is pre-calcined. In another feature, the slag material is a substantially acrystalline blast furnace slag. The glassy slag can be combined with lime-containing feedstock and sweeteners depending upon the desired clinker chemistry. For example, in a specific embodiment the slag can be provided in a 1:3 ratio relative to the lime-containing feedstock. The slag can be provided in its vitrified state obtained from the metal smelting process, although optional pre-drying of the slag may improve overall performance in certain circumstances.
It is believed that the acrystalline high-energy state of the vitrified slag leads to an exothermic heat of formation of the pyroprocessing products C2S and C3S. This exothermic phenomenon can lead to an optimization or a reduction in fuel consumption required to maintain acceptable temperatures within the kiln. Similarly, it is believed that the acrystalline high-energy state of the slag maximizes clinker production relative to the input feedstock.
One embodiment of the invention contemplates a method for the production of cement clinker in a heated rotary kiln having a feed end and a discharge end, the kiln having at least a clinkering zone between the feed end and the discharge end. In this embodiment, the method comprises the steps of: introducing a stream of feedstock material containing lime into the feed end of the kiln; introducing a vitrified slag into the kiln, preferably at or upstream of the clinkering zone; mixing the vitrified slag with the stream of feedstock material, preferably at or prior to the clinkering zone; and conveying the vitrified slag and feedstock material through the clinkering zone to the discharge end of the kiln.
In a specific embodiment, the vitrified slag is a vitrified blast furnace slag. The vitrified slag can have a glass content of at least 90% (ninety percent), and at least 75% (seventy-five percent) in certain embodiments. The vitrified slag can be pre-calcined before introduction into the kiln. In some embodiments, the vitrified slag is introduced at the feed end of the kiln, and can be mixed with the feedstock prior to introduction into the kiln. In other embodiments, the vitrified slag can be added at other points along the kiln provided that the vitrified slag passes through the clinkering zone with the feedstock. In a further aspect of the invention, the method includes the step of selecting the vitrified slag so that the chemistry of the vitrified slag is substantially similar to the desired chemistry of the cement clinker.
In another embodiment of the invention, a method is provided for the production of cement clinker in a heated rotary kiln having a feed end and a discharge end, the kiln having at least a clinkering zone at a predetermined clinkering temperature between the feed end and the discharge end. In this embodiment, the method can comprise the steps of: introducing a stream of feedstock material containing lime into the feed end of the kiln; heating the kiln from a heat source outside the kiln to produce the clinkering temperature in the clinkering zone; introducing a vitrified slag into the kiln upstream of the clinkering zone; mixing the vitrified slag with the stream of feedstock material prior to the clinkering zone; reducing the heat generated by the heat source in relation to the vitrified slag introduced into the kiln to maintain the predetermined clinkering temperature; and conveying the vitrified slag and feedstock material through the clinkering zone to the discharge end of the kiln.